Photosensors or photodetectors are sensors of light or other electromagnetic energy. There're several varieties: Optical detectors, which are mostly quantum devices in which an individual photon produces a discrete effect. Chemical detectors, such as photographic plates, in which a silver halide molecule's split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly. Photoresistors or Light Dependent Resistors (LDR) which c… (More on Photodetector)